1. Field of the Invention
The present invention relates to a secondary battery.
2. Description of the Related Art
Metallic cans have been mainly used for outer enclosures of secondary batteries. However, the number of applications of secondary batteries requiring a weight reduction such as cellular phones, notebook personal computers and automobiles is drastically growing in recent years. Therefore, so-called laminated batteries are starting to be used, using a laminated film which is relatively light-weighted and has a degree of freedom in molding for an outer enclosure to hermetically seal battery elements such as an electrode, electrode active material and electrolyte. Since the laminated film is flexible, a laminated battery is normally fixed to a case molded of resin, metal or the like and used as a battery module. Furthermore, when at least one of the laminated batteries is set up in one module, it is a general practice that the laminated batteries are tightly packed into the module so as to contact each other and improve an energy density per volume.
On the other hand, when the secondary battery in operation is overcharged or when an ambient temperature of the secondary battery becomes high for some reasons, a gas is generated due to decomposition of its electrolyte on the electrode. Since laminated batteries are generally tightly packed into the module as mentioned above, when a gas is generated and the secondary batteries swell, the case may be broken and the gas or electrolyte may leak out. Furthermore, when the gas generated accumulates around the electrode, this may prevent smooth exchange of lithium ions or the like and may thereby impair charge/discharge reaction.
In order to solve the problems caused by generation of a gas inside the battery, for example, JP2000-100399A discloses a laminated battery in which part of a heat-sealed portion of a laminated outer enclosure is heat-sealed at a lower temperature than the rest to create a portion with weak sealing strength and provides this portion with a function as a safety valve.
JP2002-56835A discloses a configuration in which part of an end of a heat-sealed portion of a laminated outer enclosure is removed from the outside. JP10-55792A discloses a configuration in which a triangular non-heat-sealed portion is provided inside part of the heat-sealed portion such that the width of the heat-sealed portion of the laminated outer enclosure that separates the interior of the battery from the outside of the battery becomes partially narrow. By providing a narrow heat-sealed portion in part of the heat-sealed portion of the laminated outer enclosure, these arts are intended to make the narrow heat-sealed portion function as a safety valve.
JP2005-203262A discloses a structure provided with a portion where peeling stress is concentrated because of swelling of an outer enclosure due to a pressure increase inside a secondary battery when a gas is generated so that this portion functions as a safety valve. JP2005-251470A discloses a structure provided with means for detecting a pressure inside a secondary battery and a heater, designed such that part of the outer enclosure sealed by heat-seal when the inner pressure increases is externally heated and melted by the heater. These arts are also intended to make the safety valve function taking advantage of an increase in the inner pressure of the secondary battery due to generation of a gas.
On the other hand, JP2005-310671A discloses a hermetically sealed battery in which a convex portion protruding toward the sealing side of an external terminal is formed on an outer enclosure. JP2006-185713A discloses a battery and a battery manufacturing apparatus capable of forming a creased portion in a fusing portion of a laminated film and thereby gradually releasing a high-pressure gas from the creased portion when the inner pressure in the battery case increases.
However, the battery disclosed in JP2000-100399A includes a safety valve structure in which the sealing strength of part of the heat-sealed portion is weakened, but stress is likely to be concentrated on the safety valve, resulting in a problem such as leakage of the electrolyte through degradation over time of the heat-sealed portion. Furthermore, it is difficult to adjust the sealing strength and not possible to accurately set an opening pressure of the safety valve.
According to the structures disclosed in JP2002-56835A and JP10-55792 of partially narrowing the width of the heat-sealed portion, the width of the heat-sealed portion of the part that functions as the safety valve needs to be set to a very narrow width, for example, on the order of 1 mm to realize a practical opening pressure. Therefore, the sealing reliability may deteriorate or an error in the width of the heat-sealed portion caused in the manufacturing process may considerably affect the opening pressure as the safety valve, and as a result, the opening pressure cannot accurately be set. One reason for such a very narrow width being required is that the thermally fusing strength (sealing strength) of the battery provided with the laminated outer enclosure is likely to be weakened at a leader part of the lead terminal for leading out the electrode, and to prevent the gas from releasing from the leader part of the lead terminal, it is necessary to open the safety valve at a lower pressure than the pressure at which the gas is released from the leader part of the lead terminal.
With the structure disclosed in JP2005-203262A using the portion where stress is concentrated due to an increase in the inner pressure as the safety valve, the sealing step becomes too complicated to manufacture a secondary battery with high reliability.
With the structure disclosed in JP2005-251470A provided with inner pressure detecting means and a heater, a mechanism for detecting the inner pressure is required, which complicates the battery circuit.
With the batteries disclosed in JP2000-100399A, JP2002-56835A, JP10-55792A, JP2005-203262A and JP2005-251470A, at least one of the single laminated batteries is tightly placed so as to contact each other, so that when the battery is prevented from swelling due to generation of a gas, the gas accumulates between the electrodes or in the vicinity of the heat-sealed portion of the laminate located apart from the safety valve, and the operation of the safety valve may be thereby retarded. Furthermore, the gas generated may accumulate in the vicinity of the electrode, which may hamper charge/discharge reaction.
With the structure disclosed in JP2005-310671A with a convex portion protruding on the sealing side of the external terminal formed on the outer enclosure, when a large volume of gas is generated due to an overvoltage or the like and an inner pressures increases, the vicinity of the terminal easily swells, the distance between the electrodes increases and it becomes difficult to exchange of lithium ions or the like which would originally take place smoothly. As a result, a phenomenon of producing a current flow locally concentrated in the laminated electrode body may occur and the battery may be further heated. Furthermore, when the gas is released outside, the electrolyte or the like may leak in the vicinity of the external terminal, triggering corrosion of surrounding wires or problems with the circuit.
With the structure disclosed in JP2006-185713A in which a crease is formed in the heat-sealed portion of the laminated film, distortion caused by contraction of thermoplastic resin is concentrated on the creased portion during heat-sealing and stress is thereby concentrated on the creased portion in an application of repeatedly receiving vibration of a cellular phone or the like and the adhesive property of the film may easily deteriorate. As a result, the electrolyte may leak out from the creased portion or outside air may enter the laminated film, and charge/discharge reaction may be affected with time.
An exemplary object of the invention is to provide a secondary battery capable of promoting charge/discharge reaction.